Airfoil blades in both compressors and turbines of gas turbine engines are subject to high, sometimes pulsating forces. Blades can experience high vibratory stresses resulting from resonance or flutter instabilities. This is particularly true for hollow blades which are used to reduce weight and/or permit internal air cooling.
External restraints such as shrouds and platform dampers have been used to control the vibration problem. Internal dampers relying on impact or dry friction have also been suggested. These have packed the blades with particles or rods, or otherwise tended to wedge the dampers. This can overload and lock the damping action.
Frictional damping inherently requires some slipping. Such slippage can be broken into macro slip and micro slip action. Macro slip is defined as substantially single point contact while micro slip is defined as a slip phenomena occurring over multiple points along the line of surface. In micro slip all points of contact are not necessarily stuck or slipping simultaneously. The pattern of local stick or slip depends on the local normal load and local deformation between the materials of the two contact surfaces.
Both micro slip and macro slip theories indicate that the vibratory response is minimized when the damper stiffness is increased. In typical applications of turbine engines to ensure high stiffness with a functionally single point contact results in a heavy damper configuration. This heavy damper configuration tends to promote sticking of the damper because of excess loading.
Those approaches which involve wedging of the damper against the surface tend to promote high loading leading to jamming or sticking of the damper rendering it ineffective.
While dampers of the prior art may have had some micro slipping along with the macro slipping, the structure was selected based on macro slip concepts. Appreciation of the micro slip phenomena and the definition of new structure to take advantage of this phenomena provides a damper of light weight, less prone to locking, and more compatible with cooling air flow within a turbine blade.